1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a ferromagnetic metal thin film. More particularly, the present invention relates to a magnetic recording medium comprising a thin film of a ferromagnetic metal which has a surface layer comprising a monomolecular layer of a saturated fatty acid or a metal salt thereof.
2. Description of the Prior Art
In place of conventional magnetic recording media of the type in which powdered magnetic materials such as powders of .gamma.-Fe.sub.2 O.sub.3, Co doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co doped Fe.sub.3 O.sub.4, and CrO.sub.2 or ferromagnetic alloy powders are dispersed in an organic binder and coated, a ferromagnetic metal thin film, which is formed using a method such as electroplating, electroless-plating, sputtering, vacuum depositing, ion plating or the like, is now attracting attention as a recording medium where no binder is used, i.e., a so-called binderless type magnetic recording medium.
As one of the requirements which are required for a magnetic recording medium for use in high density recording, it has been theoretically and experimentally proposed to render its coercive force larger and its thickness thinner. Since the thickness of the non-binder type magnetic recording medium can be easily made thinner to the extent of a thickness of about 1/10th the thickness of the binder type magnetic recording medium and the saturation magnetic flux density of a non-binder type magnetic recording medium is large, hopes for the non-binder type magnetic recording medium are high. Significant problems which are encountered in a non-binder type magnetic recording medium, i.e., a magnetic recording medium comprising a ferromagnetic metal thin film, are corrosion, abrasion resistance, and travelling stability. The magnetic recording medium is subjected to a high speed relative motion with a magnetic head in the course of recording, reproduction, and erasure of a magnetic signal. In this case, the travelling must be effected smoothly and in a stable manner, and the abrasion and breakage of the magnetic recording medium due to the contact thereof with the head must be eliminated. Furthermore, a reduction in or loss of the signal recorded due to the variation with time due to corrosion and the like during the storage must be eliminated.
Since few ferromagnetic metal layers by themselves are able to endure the severe conditions encountered in the course of the magnetic recording and reproduction, various protective layers are provided on the surface of the ferromagnetic metal layer. The formation of the protective layer by electroplating of rhodium has been commercialized to some extent. In addition, a method comprising coating a lubricant; a method comprising oxidizing the surface of a ferromagnetic metal thin film containing cobalt by allowing the thin film to stand at suitable temperatures and humidities where such a cobalt containing ferromagnetic metal is used as a magnetic substance (e.g., as disclosed in U.S. Pat. No. 3,460,968); a method comprising contacting a magnetic alloy thin film with nitric acid, subjecting the thin film to a heat-treatment to thereby form an oxidized layer on the surface thereof, and permeating lubricant into the oxidized layer (e.g., as disclosed in U.S. Pat. No. 3,719,525); a method comprising vapor-depositing chromium on the surface of a ferromagnetic metal thin film in a suitable vacuum and forming a mixed layer of chromium and chromium oxide (e.g., as disclosed in U.S. Pat. No. 3,498,837); and the like are known.
By providing a protective layer using the above described methods, the corrosion resistance, abrasion resistance, and durability are improved, but the travelling property is not satisfactorily improved. The creak which takes place when a magnetic tape comprising a ferromagnetic metal thin film is applied to VTR (video tape recording) (which means that the travelling of the magnetic tape becomes unstable because of the friction of the tape with the drum of the VTR) cannot be removed. Furthermore, the thickness of the above protective layer approaches at least 0.1 .mu. and, in some cases, up to 1 .mu. depending upon the method of forming the protective layer. Thus, the clearance between the magnetic head and the magnetic recording layer is large and high density magnetic recording is rendered impossible due to the so-called spacing-loss, which results in the loss of an important feature of the non-binder type magnetic recording medium.